This invention relates to devices that require the transfer of energy from a power source apparatus outside the body to an implanted medical device apparatus located inside the body, and more particularly to the transmission of information from the implanted apparatus to the power source apparatus, for example, to regulate the power supplied by the power source apparatus.
A transcutaneous energy transfer system (TETS) provides a means of transferring electrical energy from an external power supply apparatus to an implanted medical device through the skin of the patient. In a typical TETS, energy is transferred by magnetically coupling an external coil, located in the external power supply apparatus, to an internal coil associated with the implanted medical device.
Implanted devices receiving power from an external source typically require a load voltage within a specified operating range. An implanted blood pump may require, for example, a load voltage of not less than twelve volts, but no more than fifteen volts. The external power supply apparatus therefore needs to deliver the appropriate amount of energy to the implanted device so that the load voltage remains within the specified operating range. The load voltage level at any given time is determined by the amount of energy being transferred from the external power supply apparatus, the efficiency of the magnetic coupling between the external and internal coils, and the load imposed by the implanted medical device. The positioning of the external coil relative to the internal coil affects the efficiency of the magnetic coupling, and if the magnetic coupling is less than optimal, the external power supply apparatus may need to transmit more energy to maintain the load voltage within the specified operating range. The magnetic coupling efficiency may also change over time because, for example, patient movement may result in a change in position of the external coil relative to the internal coil, or because of the presence of an electromagnetic signal interfering with the efficiency of the magnetic coupling. Of course, fluctuations in the operation of the implanted device occur, and thus the load imposed by the medical device typically will vary over time.
To regulate the load voltage level, a feedback signal indicating the present load voltage level may be communicated from the implanted apparatus to the external power supply apparatus. One known method of providing this feedback information involves the transmission of radio frequency (RF) signals from the implanted device to the external power supply apparatus, which requires RF transmitters and receivers. Another known method involves transmitting an infrared signal with the feedback information through the patient""s skin. Yet another method is to add coils to the external apparatus and the implanted apparatus, in addition to the power transfer coils, to transmit the feedback signal from the implanted device to the external power supply apparatus.
In addition to feedback information being transmitted to regulate the load voltage level, other information may also be transmitted from an implanted medical device. For example, other information that may be transmitted may include telemetry or other logic information such as, in the case of a blood pump being the load-generating device, an indication that the pump is operating properly or is in fault, that an internal battery is fully charged or not, or whether the pump is operating on main or redundant components.
The invention, in one general aspect, features a medical TETS that provides feedback information from an implanted medical device to an external power source using the same coils over which power is transferred from the external power source to the implanted medical device.
In an aspect, an implantable device that requires electrical power from an external power source for operation has an inductor-capacitor (LC) tank circuit that receives power from the external source and delivers the power to a load-generating device. Feedback circuitry senses a load voltage level of the power delivered to the load-generating device and produces a feedback signal indicative of the load voltage level. The feedback signal is superimposed on the LC tank circuit for communication to the external power source.
The feedback signal, in one implementation, comprises a series of pulses at varying frequencies indicative of the load voltage level. The pulses may be generated at varying frequencies within a specified range with a center frequency, for example, one kilohertz. The center frequency may indicate that the load voltage level is the desired level and no adjustment needs to be made to the amount of power the external power source is delivering, while frequencies above and below the center frequency may indicate that the amount of power the external power source is delivering needs to be adjusted to bring the load voltage level back to the desired level.
The feedback circuitry may have a voltage-to-frequency converter. The voltage-to-frequency converter may include an error amplifier that compares the load voltage level to a reference voltage and generates a difference signal, and a controller to receive the difference signal and generate the feedback signal. The feedback circuitry may also have a switch that receives the feedback signal, and that is coupled with the LC tank circuit so that during the time the feedback signal is pulsed high the switch closes and the LC tank circuit is short-circuited to superimpose the feedback signal on the LC tank circuit.
The implanted medical device may also include a rectifier that includes first and second diodes having a forward bias directed toward a positive terminal of the load-generating device. The first diode may be coupled to a first terminal of the LC tank circuit and the second diode coupled to a second terminal of the LC tank circuit. The rectifier may also have first and second transistor switches whose current conducting terminals are coupled, respectively, between the first terminal of the LC tank circuit and negative terminal of the load-generating device and between the second terminal of the LC tank circuit and the negative terminal of the load-generating device. The first and second transistor switches may serve as the switch, each receiving the feedback signal at their gates and being activated by pulses in the feedback signal.
In another aspect, the invention features an external power source for an implantable device. An electrical power source is provided. A pulse-width modulator and driver generates a power signal that is transmitted over an LC tank circuit. A sensor senses a feedback signal indicative of a load voltage level that is provided by the implantable device through the LC tank circuit. The external power source varies the amount of power delivered to the implantable device in response to the feedback signal.
The external power source may also include a decoder that receives the sensed feedback signal, determines the frequency of the pulses in the feedback signal, and generates a voltage signal indicative of the frequency of the pulses in the feedback signal. The feedback signal sensed by the external power source may, as discussed previously, comprise a series of pulses, the frequency that the pulses are generated being indicative of the load voltage level. The power signal may be a rectangular pulse whose duty cycle varies the amount of energy transferred to the implantable device. The frequency range of the feedback signal may differ from the frequency of the power signal so that in the external power source, the power signal may be filtered to extract the feedback signal. The external power source may also include a tuning circuit that monitors a voltage component and a current component of the power signal so that the signal components have an ideal timing relationship.
In another embodiment, the external power source may also include a circuit that senses the rate of change of current through the primary coil. In this embodiment, the feedback signal includes, in addition to a pulsed feedback signal of the type described previously, another signal that is indicative of the rate of change of current through the LC tank circuit. The circuit that senses the rate of change of current through the coil may be a resistor-capacitor (R-C) circuit that receives a current through it that is indicative of the current through the coil. The R-C circuit has a capacitor with a voltage thereon indicative of the rate of change of current through the coil.
In a further aspect, the invention features a medical system with transcutaneous energy transfer. The medical system includes an external power source having a power driver and an LC tank circuit primary coil. An implantable device requires electrical power from an external power source for operation. The implantable device also includes an LC tank circuit that receives power from the external power source, and a load-generating device to which the received power is delivered. Feedback circuitry in the implantable device senses a load voltage level of the power delivered to the load-generating device, produces a feedback signal indicative of the load voltage level, and superimposes the feedback signal on the implantable device""s LC tank circuit for communication to the external power source.
The external power source of the medical system may include a sensor that senses the feedback signal communicated from the implantable device. The external power source may also include a decoder that receives the feedback signal sensed by the sensor, determines the frequency of the pulses in the feedback signal, and generates a voltage signal indicative of the frequency of the pulses. A pulse-width modulator and driver that receive the voltage signal and generate a power signal transferring a desired amount of energy to the implantable device may also be included. The external power source may also include a tuning circuit that monitors a voltage componet and a current component of the power signal so that the signal components have an ideal timing relationship.
The feedback signal may comprise a series of pulses, the frequency that the pulses are generated being indicative of the load voltage level. The power signal may be a rectangular pulse whose duty cycle varies the amount of energy transferred to the implanted device. The frequency range of the feedback signal may differ from the frequency of the power signal so that in the external power source, the power signal may be filtered to extract the feedback signal. The medical system may include feedback circuitry including an error amplifier that receives the load voltage level, compares the load voltage level to reference voltage, and generates a difference signal indicative of the load voltage level. A controller that receives the difference signal and generates the feedback signal may also be included.
The implantable device of the medical system may further include feedback circuitry comprising a switch that receives the feedback signal and is coupled to the implantable device""s LC tank circuit so that during the time the feedback signal is pulsed high the switch closes and the LC tank circuit is short-circuited to superimpose the feedback signal on the LC tank circuit. The implantable device may further comprise a rectifier that includes first and second diodes having a forward bias directed toward a positive terminal of the load-generating device. The first diode may be coupled to a first terminal of the LC tank circuit and the second diode coupled to a second terminal of the LC tank circuit. The rectifier may also have first and second transistor switches whose current conducting terminals are coupled, respectively, between the first terminal of the LC tank circuit and negative terminal of the load-generating device and between the second terminal of the LC tank circuit and the negative terminal of the load-generating device. The first and second transistor switches may serve as the switch, each receiving the feedback signal at their gates and being activated by pulses in the feedback signal.
In another aspect, the invention provides an implantable device requiring electrical power from an external power source for operation. The implantable device has an LC tank circuit that receives power from the external power source, and a load-generating device to which the received power is delivered. Circuitry in the implantable device senses a condition, produces a signal indicative of the condition, and superimposes the signal on the LC tank circuit for communication to the external power source.
In various embodiments, the signal being superimposed on the LC tank circuit may be one or more pulses. The load-generating device may be, for example, a blood pump, in which case the condition being sensed may be whether the blood pump is in an operating state or a fault state, whether the blood pump is operating on primary components or redundant components. In the example of a blood pump or other examples, the condition being sensed may be a charge condition for an internal battery. The signal-producing circuitry in the implantable device may include, as was the case with the feedback of load voltage information, a switch that receives the signal and is coupled with the LC tank circuit so that during the time the signal is pulsed high the switch closes and the coil is short-circuited to superimpose the signal on the LC tank circuit. The implantable device may also include the previously described circuitry that performs both rectifier and switch functions.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.